1. Field of the Invention
The invention generally relates to the enhancement of transistor performance, and more particularly to the enhancement of transistor performance by creating a desired stress in the transistor channel region.
2. Background of the Invention
As semiconductor devices shrink, carrier mobility has become a roadblock for improved device speed. Studies have shown that electron mobility can be increased significantly by creating tensile stress in a transistor channel, and hole mobility can be improved by creating compressive stress. Thus, to improve the characteristics of a semiconductor device, tensile and/or compressive stresses are created in the channel of the n-type devices (e.g., nFETs) and/or p-type devices (e.g., pFETs). Additionally, the higher the stress in the channel, the higher mobility improvement which may be realized.
The channel stress may be induced by a Si3N4 or nitride film deposited on top of the transistor. The induced stress in the channel has the same sign (tensile or compressive) with that of the nitride film. However, the induced channel stress is only a fraction of the nitride film in the magnitude. Typical stresses are about 12 GPa for tensile stresses, and about 12.5 GPa for compressive stresses, hence, the maximum strain effect is limited.
For example, a related art transistor having a stressed transistor channel includes a silicon substrate having a gate oxide formed on its upper surface. Next, a polysilicon gate is deposited on the gate oxide. Offset spacers or gate sidewalls are formed adjacent both sides of the polysilicon gate. The gate sidewalls may be used to form a proper ion implanted extension structure within the silicon substrate. Additionally, Si3N4, spacers are formed fully on each sidewall of the polysilicon gate. It should be noted that the Si3N4, spacers are typically attached to the gate sidewalls prior to ion implantation. Next, ion implantation is used to form source/drain regions within the silicon substrate on both sides of the polysilicon gate.
Further processing includes forming substrate salicide regions proximate to the source/drain regions and a gate salicide region formed on the polysilicon gate. The substrate and gate salicide regions may include CoSi or NiSi. The substrate and gate salicide regions are formed self-aligned to the source/drain regions and polysilicon gate regions.
Next, a highly stressed Si3N4 or nitride film is uniformly deposited over the silicon substrate, source and drain regions, offset spacers, and polysilicon gate with a same thickness. The stressed nitride film is deposited as a conformal layer, including being deposited over the sidewall spacers. The nitride film causes stress in the silicon substrate, including the transistor channel region through a mismatch in the crystal lattice structures of the silicon substrate and the nitride film. The induced stress in the transistor channel region is proportional and of the same sign (tensile or compressive) with the stress in the nitride film. The magnitude of the induced stress is a fraction of the stress in the nitride film.
It should be noted that in the device described above, the induced stress in the transistor channel region is a small fraction of the stress of the nitride film. Because the improvement in carrier mobility increases with increased stress magnitude in the transistor channel region, a method to produce higher stress in the transistor channel region would further improve transistor performance.